FIGS. 1A-1C depict a so-called phase shift type TOF system 50′ that can measure depth distance Z between the TOF system and a target object. In such system, distances Z to a target object are detected by emitting active light modulated optical energy Sout, typically 10 MHz to 100 MHz, of a known phase, and examining phase-shift in the reflected optical signal Sin from the target object 52. Exemplary such phase-type TOF systems are described in several U.S. patents assigned to Canesta, Inc., assignee herein, including U.S. Pat. No. 6,515,740 “Methods for CMOS-Compatible Three-Dimensional Imaging Sensing Using Quantum Efficiency Modulation”, U.S. Pat. No. 6,906,793 entitled Methods and Devices for Charge Management for Three Dimensional Sensing, U.S. Pat. No. 6,678,039 “Method and System to Enhance Dynamic Range Conversion Useable With CMOS Three-Dimensional Imaging”, U.S. Pat. No. 6,587,186 “CMOS-Compatible Three-Dimensional Image Sensing Using Reduced Peak Energy”, U.S. Pat. No. 6,580,496 “Systems for CMOS-Compatible Three-Dimensional Image Sensing Using Quantum Efficiency Modulation”. FIG. 1A is based upon the above-referenced patents, e.g. the '186 patent.
In FIG. 1A, exemplary phase-shift TOF depth imaging system 50′ may be fabricated on an IC 54 that includes a two-dimensional array 56 of single-ended or differential pixel detectors 58, and associated dedicated circuitry 60 for processing detection charge output by the associated detector. IC 54 preferably also includes a microprocessor or microcontroller unit 62, memory 64 (which preferably includes random access memory or RAM and read-only memory or ROM), a high speed distributable clock 66, and various computing and input/output (I/O) circuitry 68. Among other functions, controller unit 62 may perform distance to object and object velocity calculations.
In system 50′, under control of microprocessor 62, optical energy source 70 is periodically energized by an exciter 76, and emits modulated optical energy toward an object target 52. Emitter 70 preferably is at least one LED or laser diode(s) emitting low power (e.g., perhaps 1 W) periodic waveform, producing optical energy emissions of known frequency (perhaps a few dozen MHz) for a time period known as the shutter time (perhaps 10 ms). Emitter 70 typically operates in the near IR range, with a wavelength of perhaps 800 nm. A lens 72 may be used to focus the emitted optical energy.
Some of the emitted optical energy (denoted Sout) will be reflected (denoted Sin) off the surface of target object 20. This reflected optical energy Sin will pass through an aperture field stop and lens, collectively 74, and will fall upon two-dimensional array 56 of pixel or photodetectors 58, often referred to herein as pixels, arranged typically in rows and columns. When reflected optical energy Sin impinges upon the photodetectors, photons within the photodetectors are released, and converted into tiny amounts of detection current. For ease of explanation, incoming optical energy may be modeled as Sin=A·cos(ω·t+Θ), where A is a brightness or intensity coefficient, ω·t represents the periodic modulation frequency, and Θ is phase shift. As distance Z changes, phase shift Θ changes, and FIGS. 1B and 1C depict a phase shift Θ between emitted and detected signals. The phase shift Θ data can be processed to yield desired Z depth information. Within array 56, pixel detection current can be integrated to accumulate a meaningful detection signal, used to form a depth image. In this fashion, TOF system 40′ can capture and provide Z depth information at each pixel detector 58 in sensor array 56 for each frame of acquired data.
As described in the above-cited phase-shift type TOF system patents, pixel detection information is captured at least two discrete phases, preferably 0° and 90°, and is processed to yield Z data.
System 50′ yields a phase shift Θ at distance Z due to time-of-flight given by:Θ=2·ω·Z/C=2·(2·π·f)·Z/C  (2)
where C is the speed of light, 300,000 Km/sec. From equation (2) above it follows that distance Z is given by:Z=Θ·C/2·ω=Θ·C/(2·2·f·π)  (3)And when Θ=2·π, the aliasing interval range associated with modulation frequency f is given as:ZAIR=C/(2·f)  (4)
In practice, changes in Z produce change in phase shift Θ but eventually the phase shift begins to repeat, e.g., Θ=Θ+2·π, etc. Thus, distance Z is known modulo 2·π·C/2·ω=C/2·f, where f is the modulation frequency. Thus there can be inherent ambiguity between detected values of phase shift Θ and distance Z. In practice, multi-frequency methods are used to disambiguate or dealias the phase shift data.
Typical time of flight (TOF) sensors require multiple image captures of different configurations to measure depth or Z-distance to a target object. Multiple TOF system acquired images from discrete points in time are then combined to yield a single depth frame. A primary source of so-called bias error results from motion of the target object over time during image acquisition. Another source of bias error is due to depth edges in space. In either case, a pixel (or an array of pixels) images more than one object and returns a single incorrect depth value. It is advantageous to maximize resolution in both space and time to minimize bias from such effects.
What is needed is a method and system whereby resolution in time and space can be maximized in a phase-based TOF system. Preferably a decision as to which parameters shall be maximized should be determinable on-the-fly.
The present invention provides such a method and system.